There are quite a few industrial processes where one requires separating between different materials comprised in a mixture of particles having essentially a similar size.
Some of the solutions proposed in the past to deal with such a problem are:
U.S. Pat. No. 5,887,803 describes a process and an apparatus for grinding and sifting a product, in which the product is embrittled by a coolant and is comminuted in a mill and fed to a sifter. According to this publication, the product is comminuted and sifted in a closed-loop process without separating agents having to be fed to the ground product prior to sifting to overcome electrostatic charging.
U.S. Pat. No. 5,755,388 discloses a waste tire treating apparatus which comprises a grinding device for grinding the waste tire into blocks of smaller volume, a granulating device for cutting and granulating the dried waste tire into granules with predetermined volume, a heating device for heating and melting the waste tire granules and a sorting device disposed under the heating device for receiving the molten rubber, the nylon fibers and the steel filaments of the waste tire. The sorting device includes a channel with circulated water flow, whereby the mixture of the rubber and nylon fibers float on top of the water, while the steel filaments precipitate onto the bottom of the channel and are separately recovered.
U.S. Pat. No. 6,308,903 describes an apparatus and method for processing used vehicle tires that separates the steel metal from the rubber and other non-metallic materials for recycling. The tire is rotated at sufficient speed to force the materials outwardly by centrifugal force. Heated blades are used to melt and cut through the non-metallic materials, dividing the tire into at least two arcs that are held together by the steel reinforcement that is not easily cut through. Electric heating apparatus heats the steel to a higher temperature than the rubber so that the forces binding the rubber are released, thereby enabling the rubber and other non-metallic materials to be flung outwardly away from the steel and the steel is then collected separately.
U.S. Pat. No. 6,325,215 discloses a method for separating elastomeric particulates from a pulverized mixture wherein a first separator assembly separates out clean fiber and passes non-fibrous particulates and residual fiber to a second separator assembly that separates out clean non-fibrous particulates. The method includes propelling the pulverized mixture under a number of separator cylinders with mixture-engaging structure and against a structure for removing non-fibrous particulates and residual fibers from the pulverized mixture. The non-fibrous particulates and residual fibers are propelled under a plurality of other separator cylinders that include centrifugally releasable mixture-engaging structures and against a structure for separating non-fibrous particulates from the residual fibers.
Such separation processes, which are not too trivial, become rather complex processes when the mixtures are in the form of fine or even ultrafine particles, as most of the common methods are not applicable to handle particles of this size range.